1. Field of the Invention
The present invention relates to a power transmission apparatus for a hybrid vehicle in which an engine output and a motor output are transmitted to a drive wheel.
2. Description of the Related Art
In a hybrid vehicle that is installed with an engine and an electric motor so that an engine output and a motor output can be transmitted to a drive wheel, the engine output is transmitted to the drive wheel via a shift mechanism. In a hybrid vehicle installed with a shift mechanism, the engine and the electric motor may be arranged such that the engine is disposed on one end portion side of a transmission input shaft of the shift mechanism and the electric motor is disposed on the other end portion side, whereby the engine and the electric motor are disposed on either side of the shift mechanism, or such that the engine and the electric motor are disposed adjacent to each other on one end portion side of the transmission input shaft. In a hybrid vehicle having a shift mechanism, the engine output and the motor output are transmitted drive wheel via the shift mechanism. In a case where the electric motor is caused to function as a generator in order to recover regenerative energy, regenerative torque from the drive wheel is transmitted to the electric motor via the shift mechanism.
Japanese Unexamined Patent Application Publication (JP-A) No. 2004-11819 describes a hybrid vehicle in which a continuously variable transmission (CVT) is used as the shift mechanism, an engine is disposed on one end portion side of a primary shaft serving as the transmission input shaft, and an electric motor is disposed on the other end portion side. In this hybrid vehicle, an oil pump is driven by an electric motor for travel to generate oil pressure used by a power transmission mechanism. A reverse rotation prevention mechanism employing a planetary gear and a one-way clutch is provided between the electric motor and the oil pump such that when the vehicle is caused to reverse by rotating the electric motor in reverse, the oil pump is driven to rotate in an identical direction to a direction for forward advancement by the reverse rotation prevention mechanism.
JP-A No. 2007-261348 describes a hybrid vehicle in which a continuously variable transmission is used as a shift mechanism, and an engine and an electric motor are disposed adjacent to each other on one end portion side of a primary shaft. When this hybrid vehicle travels by transmitting an engine driving force to a drive wheel via the continuously variable shift mechanism, the driving force is prevented from being transmitted in reverse to the electric motor by a one-way clutch, and as a result, driving force loss caused by drag in the electric motor can be prevented from occurring.
In the conventional hybrid vehicles described above, the engine output and the motor output are transmitted to the drive wheel via a shift mechanism, and therefore power transmission loss in the shift mechanism is inevitable. When the electric motor is caused to serve as a generator order to recover regenerative energy, regenerative torque from the drive wheel is transmitted to the electric motor via the shift mechanism, leading to further power transmission loss.
Examples of the continuously variable shift mechanism serving as the shift mechanism include a belt drive type mechanism and a traction drive type mechanism. In a belt drive system, a primary shaft having a primary pulley and a secondary shaft having a secondary pulley are provided, and groove widths of the respective pulleys are set to be variable. A power transmission element such as a belt is wound between the two pulleys, whereby rotation of the primary shaft is varied continuously via the power transmission element and transmitted to the secondary shaft. In a traction drive system, a primary shaft having an input disk and a primary shaft having an output disk are provided, a toroidal surface is formed on each of the disks, and a power roller is disposed between the toroidal surfaces as a power transmission element.
In the continuously variable shift mechanism, a tightening force oriented toward the belt or other power transmission element must be exerted on the pulleys using a hydraulic pump during power transmission. Likewise in a stepped shift mechanism, oil pressure supplied from a hydraulic pump to frictional engagement elements such as a clutch and a brake for switching a gear position in order to drive the clutch and so on. Hence, in a power transmission apparatus for a hybrid vehicle employing a shift mechanism, a hydraulic pump for driving the shift mechanism must be driven, and therefore, in addition to the power transmission loss occurring the shift mechanism, power loss inevitably occurs when driving the hydraulic pump.